This study will examine the development of the EEG sleep rhythm in the "healthy low birthweight neonate (<1500 grams) from birth through the expected date of delivery. Maturational similarities and differences in neurophysiological development will be assessed between preterm infants of increasing gestational ages up to term, compared to a control group of infants whose birthweights are appropriate-for-gestational-age. EEG sleep state will be analyzed in terms of its interaction with rapid eye movements (REMs), motility, and arousal to assess ultradian (less than 24-hour) cyclicity in the developing brain of the neonate. These signals will be evaluated visually and with computer assistance. Three specific hypotheses will be examined: 1) EEG sleep organization in healthy preterm neonates will follow a predictable maturational pattern as they reach their full-term age; 2) EEG sleep state, motility, and arousal of premature neonates corrected to full-term age will differ from normal full-term infants with respect to an altered sleep cycle, increased number and types of body movements, and increased duration of arousals; 3) Maturational relationships between the ultradian sleep rhythm of the neonate and other ultradian rhythms of REM, arousal, and motility are established by a corrected term age, and can be expressed by computer analysis in terms of spectral power, coalescence, and periodicity. These mathematical models reflect the degree of neurophysiological organization of the central nervous system at birth. The establishment of normative EEG-sleep data for the "healthy" premature neonate as he/she matures to term is essential before accurate comparisons can be made with sick premature neonates who are at risk for neurologic sequelae. Mathematical algorithms of neonatal sleep organization (i.e. spectral power, coalescence, and periodicity) from the present study of healthy premature neonates can be compared with premature neonates at high-risk. The study will be carried out using monitoring systems already in place. These systems include both paper and synchronized video- EEG monitoring, and portable computer equipment for data reduction and analysis. This will be done without disruption of the patient's in-patient medical care. A Neonatal Follow-Up Clinic is already in operation to assist in the neurodevelopmental assessment of the study patients through age five, as specified by the study design.